Microporous filter media are employed for removing ultra-fine or microscopic particles from the medium from which they are suspended. As examples of particulate media which may be separated by such filters from the fluid environment in which they are disposed, mention may be made of bacteria, colloidal substances and the like .
Surface-type filters, while capable of efficiently filtering the materials of a relatively small size suffer from a deficiency or lack of uniformity of pore size, fragility as the pore size becomes more uniform and a relatively low percentage of open area. The term "open area", as used herein, is intended to refer to the voids or pores or openings in the filter unit in contrast to the closed or land areas intermediate the pores. It will be readily seen that the efficiency of a filter would increase with an increase in open area.
The term "surface-type filters", as used herein, is intended to refer to thin films having pores which extend from one surface to the other and function by trapping material on the upstream side of the filter. This is in contradistinction to depth-type filters which are relatively thick, posses pores of significant length and function by trapping materials within the filter element.
Copending application of Arthur M. Gerber, Ser. No. 601,857 filed Apr. 19, 1984 (common assignee) is directed to a surface-type filter having substantially uniform pore size of less than 10 micrometers and an open area of at least 10%. The filters are formed by embossing uniform holes in an embossable polymer. Thus, in one embodiment a layer of embossable polymer is carried on a temporary support which provides a base which can be penetrated by the embossing tool to insure that the pores in the embossable polymer extend from one surface to the other.